I recently restarted the master node and found that no shards have been allocated to it. Master node version is 7.20 and data nodes are 7.21. Is that an issue? How do I go about updating the Master node version (installed as rpm)?
Yes, that is an issue. All nodes need to use exactly the same version as shards on a newer version can not be allocated to a node with a lower version.
How do I go about updating to 7.21?
Have a look at the docs as they describe the process.